34 Chapter 2. What’s inside cells[[Student version, December 8, 2002]]
return many times to the interactions responsible for protein structure and function.- Most cells can reproduce by mitosis, a process of duplicating their contents and splitting
in two. (A few types instead create germ cells by meiosis; see Section 3.3.2.) - All cells must maintain a particular internal composition, sometimes in spite of widely
varying external conditions. Cells generally must also maintain a fixed interior vol-
ume (see Chapter 7). - By maintaining concentration differences of electrically charged atoms and molecules
(generically called “ions”), most cells also maintain a resting electrical potential
difference between their interiors and the outside world (see Chapter 11) Nerve and
muscle cells use this resting potential for their signaling needs (see Chapter 12). - Many cells move about, for example by crawling or swimming. Chapter 5 discusses
the physics of such motions. - Cells sense environmental conditions for a variety of purposes:
1.Sensing the environment can be one step in a feedback loop that regulates the
cell’s interior composition.
2.Cells can alter their behavior in response to opportunities (such as a nearby
food supply) or hardships (such as drought).
3.Single cells can even engage in attack, self-defense, and evasive maneuvers upon
detecting other cells.
4.The highly specialized nerve and muscle cells obtain input from neighboring
nerve cells by sensing the local concentration of particular small molecules, the
neurotransmitters,secreted by those neighbors. Chapter 12 will discuss this
process.- Cells can also sense their owninternalconditions as part of feedback and control loops.
Thus for example an abundant supply of a particular product effectively shuts down
further production of that product. One way feedback is implemented is by the
physical distortion of a molecular machine when it binds a messenger molecule, a
phenomenon called “allosteric control” (see Chapter 9). - As an extreme form of feedback, a cell can even destroy itself. This “apoptosis” is a
normal part of the development of higher organisms, for example removing unneeded
neurons in the developing brain.
2.1.1 Internal gross anatomy
Paralleling the large degree of overlap between thefunctionsof all cells, we find a correspondingly
large overlap between their gross internalarchitecture:Most cells share a common set of quasiper-
manent structures, many of them visible in optical microscopes. To see some of the fine substructure
below, however, we need to use the electron microscope, an instrument that gives better resolution
but requires killing the cell.